1. Field of the Invention
The present invention relates generally to fiber/resin composites and a method of making same. In a specific aspect, the present invention relates to articles comprising filament arrays of parallelly aligned, laterally continuous filaments, such as are formed by filament winding, braiding, and pultrusion processes.
2. Description of the Related Art
In the field of composite materials, a variety of fabrication methods and techniques have come into usage for producing fiber-reinforced resin matrix materials.
These include prepreg manufacture, lay-up techniques, sheet molding of chopped fiber laminates, resin transfer molding, and reinforced reaction injection molding, all of which involve the fabrication of fiber-reinforced resin materials using short fibers.
Continuous filament processes also have evolved which are adapted to automated production of filament-reinforced resin articles. The continuous fiber processes include filament winding, wherein the filament in the form of discrete strands or roving is coated with a resin then wound on a mandrel at a predetermined angular orientation and winding thickness to yield articles having high strength when the resin borne on the filament is cured. Another continuous filament process is braiding, in which a plurality, e.g., as many as 144 separate fiber tows are interwoven to form tubular products. Continuous filaments may also be employed in pultrusion processes, wherein a plurality of filament strands or rovings are passed under tension through a resin bath to apply a resin coating thereto, following which the filament is drawn through a preformer, or initial forming die, which imparts a selected cross-sectional shape to the fiber array. The initially shaped fiber array is next passed through a heated die with constant cross-sectional area, by which the resin is cured, with the resulting rigid formed article being withdrawn and conveyed to a cut-off saw or other severing apparatus to form discrete product articles. The pultrusion process is conducted under continuous tension, by means of a puller or other drawing means which pulls the filament through the constituent unit step in the process systems.
A wide variety of composite product articles are formable by continuous filament processes such as filament winding, braiding, and pultrusion methods, including golf clubs, tennis rackets, pressure vessels, fishing rods, gas tanks, drive shafts, aerospace control linkages, sliding bearings, helicopter rotars, pipes, tubes, solid rods, missile launchers, artillery, bazookas, gun barrels, and various other structural members and component parts utilized in the aerospace, automotive, appliance, and consumer product industries.
Considering filament winding in grater detail, the mandrel which is employed either may be reusable in character or it may be employed as a constituent part of the article to be formed (the mandrel being removable with the product article from the forming machinery in the latter case). Alternatively, the mandrel may be of a temporary character, providing a winding substrate during the filament winding operation, but being subsequently removed by dissolution or decomposition to provide an interior cavity in the filament wound article. Temporary mandrels may be formed for example of eutectic salts, plastics, mixtures of sand and polyvinyl chloride, etc., for such purpose.
The resins which are employed in filament winding, braiding, and pultrusion operations must exhibit constant viscosity and long pot life in the process systems in which they are employed. Constant viscosity is required in order that coating of the resin on the continuous filament is highly uniform in character, as required to achieve substantially uniform, e.g., isotropic, properties in the final product article.
If viscosity changes appreciable during the filament winding, braiding, or pultrusion operation, the applied resin thickness may change significantly, and such thickness change may in turn result in (i) localized stresses or discontinuities in the final product article, (ii) product articles which are not within required dimensional tolerance specifications, and (iii) inadequate curing of the resin. In addition, the tensional forces on the resin impregnated filaments being processed will significantly increase with increases in the resin viscosity, to such extent that the filament becomes highly susceptible to snapping, i.e., tensionally breaking.
Long pot life of the resin is necessary, particularly in filament winding and braiding, where processing times may be on the order of hours. Since the resin is continuously being applied to the filament in these processes, the resin bath or other source of the resin must be continually replenished with resin coating material, and it is therefore necessary that the resin not "set up" or gel in the source bath or other source container and applicating means.
The standard resins which have been employed in the above-described continuous filament processes, as well as in fiber/resin composites manufacture generally, are one-part heat curing epoxy resins, or two-part ambient temperature cure or heat cure epoxies. These thermal cure and ambient cure resins have associated deficiencies which have specifically limited the utility of filament winding, braiding and pultrusion processes, and are also frequently disadvantageous in other fiber/resin composite manufacturing operations.
In filament winding applications, the mandrels employed may be of materials which are adversely affected by heat so that thermal curing resins cannot be usefully employed. An example is the fabrication of rocket motors in which resin-bearing filament is wound onto a solid rocket fuel body. In such applications, ambient cure resins must be employed. However, since the filament winding operation may take upwards of 6 hours and since viscosity cannot vary during this period, a long pot life ambient cure resin is essential, and consequently the filament wound body must be rotated until full cure of the ambient cure resin is achieved, which in the case of epoxy resins conventionally employed is upwards of 4 days. Continuous rotation of the mandrel and filament winding is essential in such case, since cessation of rotation would result in the viscous resin sagging under gravitational forces, resulting in a resin-rich lower portion of the product article and a resin-poor upper portion of same.
Similar problems of resin sag and long ambient cure times frequently arise in filament braiding operations.
In pultrusion processes, ambient cure resins are typically not advantageous due to the large volume of storage space which would be required during curing of the pultruded article under ambient, e.g., room temperature, conditions. Accordingly, it has been common practice to utilize thermally cured resins in the pultrusion process which are cured concurrently with the passage of the fiber array through the forming die, typically by a heated die or a heater associated therewith.
Nonetheless, such use of thermally cured resins in the pultrusion process is disadvantageous when solvent-based thermally curable resin formulations are employed, since the linear speed of the drawn fibers needs to be fairly high in order to achieve economy in the continuous filament processing operation. Accordingly, it is desirable to cure the formed fiber array very quickly, by the imposition of suitably high temperature. This in turn raises the problem that a high localized heating intensity imposed on the formed filament array may result in rapid loss of the solvent as well as other volatile components in the resin, causing pinholes, blisters, and other localized discontinuities in the applied coating which adversely affect the strength, impact resistance, and other physical properties, as well as the aesthetic characteristics of the product article. Typically, linear speeds of the drawn filaments in conventional commercial pultrusion processes are on the order of from about 0.5 to 2 feet per minute, to accommodate an elevated temperature curing step of approximately 10 minutes (retention time in the heated die/curing oven).
Accordingly, it would be a significant advance in the art to overcome the above-described difficulties associated with the filament winding, braiding, and pultrusion processes, in a manner which as specifically applied to filament winding of heat-sensitive cores or mandrel members would obviate the use of long rotation periods heretofore necessary for curing of ambient cure resins, which in filament braiding operations reduces the problem of resin sag and extended curing times, and which in specific application to pultrusion processes, permits the processing rate in the process system to be measurably increased.
Accordingly, it is an object of the present invention to provide an improved process for forming fiber/resin composites.
It is a further object of the invention to provide an improved process for filament winding, braiding and pultrusion, which overcomes the above-described deficiencies of the prior art practice of these processes.
It is another object of the invention to provide filament wound, braided, and pultruded articles which are readily and economically formed, and which are rapidly processed for subsequent handling, packaging, or other processing operations.
Other objects and advantages of the present invention will be more fully apparent from the ensuing disclosure and appended claims.